There is a whole range of Bitcoin private keys iterated from the beginning to the end.
Now how it works: Private key is a simple integer between 1 and 1.157.920.892.373.161.954.235.709.850.086.879.078.528.375.642.790.749.043.826.051.631.415.181.614 inclusive (slightly less than 2256 ). Yes, this is an unfathomably large number. It is approximately 1077 in decimal. The visible universe is estimated to contain 1080 atoms. That has a minimal chance, near zero, to repeat randomly.We convert private keys to WIF (Wallet Import/Export Format) for a convenient using. For more details about getting WIF and Bitcoin address, read at About wallet page.